The invention relates generally to wind towers and more specifically to the design of flangeless wind towers.
Wind Turbine towers are frequently constructed of multiple units of tubular cross-section components. Tubular support structures have many and varied types of application. Some types of support structures exist where equipment is supported at elevated heights from the ground. These multi-section support structures may be tall and carry operating equipment of various weights at the top, thereby subjecting the joints between the multiple sections in these structures to high stress. The tubular, multi-section support structures may be used in many applications, including cellular phone towers, radar towers, and wind towers.
Wind turbine support towers are large structures, sometimes extending to significant heights to accommodate large wind turbine rotor blades and to strategically place the rotor blades within a wind path. For example, a typical tower may have a height as high as about 100 meters (m). Such a tower may include multiple sections, often a bottom, a middle and a top section. The length and number of individual sections may vary according to the application and height of the structure. The cross section for many such towers is generally circular and may be tapered at upper levels. Tapering may be achieved by use of individual tubular segments, the tubular segments tapered as right conical sections along an axial, vertical, direction.
Mounted on top of the support tower for wind turbines is a nacelle. The nacelle houses, or encloses, the equipment and components of the wind turbine and includes hubs for the wind turbine blades and the power train including the bearing, gearbox and electrical generator for the wind turbine. Typically, a yaw bearing is mounted to the top section of the tower. A bed plate supporting all of the weight of the power train rotates on the yaw beating, allowing wind turbine controls to rotate the nacelle to better position the blades wind respect to the wind direction for optimizing performance.
Certain prior art tubular wind turbine structures were fabricated with tubular sections of welded segment assemblies with flanges welded on the ends of the tubular sections. A top closure flange and a bottom closure flange of adjacent sections are then bolted together along a bolt line to assemble the sections together. On-going problems with the exemplary wind turbine support tower include weld cracking, buckling of the tower wall, loosing fastener pre-load, maintenance requirement, flange distortion and bolt failure during tower flexure, each contributing to the life cycle cost of the tower. Moreover, the heat generated during the flange welding process distorts the flanges out of their bolting plane creating joining issues that include excessive bolt preload and weld residual stress.
U.S. Publication 2008/0041009 A1, dated Feb. 21, 2008 by Cairo et al., discloses a flangeless wind turbine tower to replace the common welded flange wind tower design and is illustrated in FIGS. 1-3. In the flangeless design, fingerplate assemblies are used to replace welded flanges on the tubular sections as the means of joining the tubular sections of the tower to build up the full height of the tower. The flangeless concept and subsequent bolted fingerplate assembly eliminates the above-described problems such as weld cracking, tower wall buckling, loosing fastener pre-load, maintenance requirement, flange distortion and bolt failure during tower flexure. The flanged tower sections undergo distortion during service loading in addition to the post-weld distortion. Thus a flangeless wind turbine tower design, incorporating fingerplates, was presented, eliminating the drawbacks of flanged wind towers. With the current wind tower flange fastening method, the type of loading is tensile, whereas with the finger plate design loading is shear mode. A friction connection (finger plate design) is the most fatigue resistant type of connection used in steel construction today. This fastening method has the advantage that once the bolt is tensioned, it never sees additional load. The bolt merely provides the normal force required for the friction between the plates to work.
FIG. 1 illustrates a side sectional view of a fingerplate assembly 10 for tubular assemblies that overcome the previously described problems, in tubular assemblies with welded flanged joints, of flange distortion after welding. The annular rings forming the end surfaces of two adjacent flangeless tubular sections 20 and 30 of the tubular assembly (without flanges) are brought together in close proximity at point 25. A space 28 between the adjacent tubular sections 20, 30 is referred to as an intercan gap. An inner fingerplate 40 is provided on an interior of the fingerplate assembly 10. The inner fingerplate 40 may be provided with a curved outer diameter matched to the curved inner diameter of the flangeless tubular sections 20 and 30. The inner fingerplate 40 is provided for connecting the inner surfaces 22 and 32 of the adjacent flangeless tubular sections 20, 30. An outer fingerplate 50 is provided on an exterior of the fingerplate assembly 10 for connecting with outer surfaces 24, 34 of the adjacent flangeless tubular sections 20, 30. The outer fingerplate 50 may be provided with a curved inner diameter matched to the curved outer diameter of the tubular sections 20 and 30. The outer fingerplate 50 is provided for connecting the outer surfaces 24 and 34 of the adjacent flangeless tubular sections 20, 30. Fastening arrays of throughholes 55 are provided on each fingerplate 40, 50 and are matched with the fastening array of throughholes 55 provided on the corresponding adjacent ends of the flangeless tubular sections 20, 30. The assembly further may further include bolts 60 and nuts 65 according to the throughhole array. However, other suitable fastening means may be utilized depending upon the particular application.
FIG. 2 illustrates an isometric view of a typical fingerplate utilizing nut and bolt fastening. The typical fingerplate 70 has an inner surface 72 and an outer surface 74. For an inner fingerplate, its outer surface is matched to the curved outer surface of the adjacent tubular sections. For an outer fingerplate, its inner surface matched to corresponding surface of adjacent tubular sections. A typical bolt throughhole array 76 is shown for connection with one tubular section and typical bolt throughhole array 78 is shown for connection with the adjacent tubular section. Fingerplate design is according to standard design practice including spacing of bolt throughholes from the edge of the fingerplate, spacing between adjacent bolt throughholes, thickness of the fingerplate, surface dimension of the fingerplate and plate material selection.
FIG. 3 illustrates a flangeless joint 90 with fingerplate assemblies 92 uniformly distributed around the periphery of a lower tubular section 96. The inner fingerplates 93 and outer fingerplates 94 and the lower tubular section 96 are shown for clarity. An tipper tubular section and fasteners are omitted for the sake of clarity. For an exemplary wind turbine support tower of about 80 m, five fingerplate assemblies may be distributed around the periphery of the adjacent sections of the wind turbine support tower. Further bolting may be employed as a fastening means for the wind turbine support tower.
A simplified representation of a scheme for bolting throughholes is shown in FIG. 3. FIG. 2 illustrates a fingerplate with a more typical throughhole array for the exemplary wind turbine support tower. For an exemplary 80 m tower, the fingerplate may have an arc dimension of about 2 m. a height of about 1 m, and a thickness of about 20 mm. The material for fingerplates may preferably include ASTM A 572 Gr 50 steel plate. Bolt throughhole arrays 76 and 78 on the fingerplates may be preferably configured in double rows applied to each adjacent section of tower for a total of about 48 boltholes per fingerplate. Diameter for the bolt throughholes may preferably be sized about 1.25 inch. Minimum spacing between the bolt throughholes may be about 5 inches. Typical bolts for the fingerplates in the 80 m tower may preferably be M36 10.9 grade bolts that are torqued to a bolt prestress of about 510 MPa (74 ksi).
While the flangeless windtower eliminated the adverse performance of the flanged wind tower, as previously described, further analysis of the flangeless wind tower concept suggests that the overall stiffness of the flangeless tower with fingerplates was low. The stiffness of the flangeless windtower could be as low as one-tenth that of the flanged tower. Low stiffness can lead to excessive tower sway, resulting in high compressive forces on tower sections and fingerplates at the joints. The high compressive forces can lead to unacceptable distortion of the tower sections and fingerplates at the joint, potentially resulting in local failure or overall structural instability due to compression induced buckling.
Accordingly, there is a need to address elements of a flangeless wind turbine tower arrangements to improve the stiffness to be comparable with the baseline flanged wind tower that it replaced.